In no particular order,
* .22lr -- Cheap guns, cheap ammo, lots of fun, readily available.
* .223/5.56mm -- Accurate, lots of different loads for different uses, flat trajectory, ammunition readily available.
* 7.62x39mm -- Good all-around caliber, readily available. I think I may get a 7.62x39mm upper for my AR soon.
And, just to stir the pot;
*6.8mm. It's the best round ever. Jesus engineered it personally. Blows up tanks, downs helicopters, shoots deer through trees, and does not over penetrate soft targets.